The present invention relates to a method for producing an electric double layer capacitor which is excellent in reliability of the performance for a long period of time and which has a high withstand voltage and a high energy density.
An electric double layer capacitor has excellent properties with respect to the output density and the long term reliability for charge and discharge cycles, but it has a problem such that its energy density is low as compared with e.g. a lithium ion secondary cell. The amount of energy to be stored in a capacitor is proportional to the capacitance of the capacitor and to the square of the applied voltage. Accordingly, the energy density can effectively be increased by increasing the withstand voltage of the capacitor.
The withstand voltage of an electric double layer capacitor is restricted basically by the electrochemical decomposition voltage of the electrolyte. When an organic electrolyte having a high withstand voltage as compared with an aqueous electrolyte, is used as the electrolyte, electrolysis is likely to take place due to impurities, particularly water, contained in the electrolyte. Accordingly, for the organic electrolyte, both the solvent and the electrolytic salt will be used as highly purified and dehydrated.
On the other hand, for the electrodes for an electric double layer capacitor, an electrode material having a high surface area is used, and when it is to be combined with the above organic electrolyte, activated carbon is usually employed as the electrode material having a high surface area. Activated carbon is a porous material having fine pores of a few nm, but it is likely to adsorb moisture in the environment, as it has a high adsorbing ability. Accordingly, activated carbon is required to be highly dehydrated during the process for producing an electric double layer capacitor.
In order to completely remove moisture from fine pores of activated carbon, it is usually required to carry out dehydration treatment at a high temperature of at least 300xc2x0 C. in vacuum or in an inert gas atmosphere. However, activated carbon particles are usually molded by means of a binder such as an organic polymer on a current collector to form an electrode, and the binder undergoes thermal decomposition by high temperature treatment of at least 300xc2x0 C. Therefore, heat treatment may usually be possible only at a temperature of not higher than 200xc2x0 C., whereby it is difficult to completely remove moisture in the activated carbon electrode.
For the purpose of increasing the energy density, an electric double layer capacitor employing an organic electrolyte is operated at a voltage of at least 2 V which is higher than the theoretical decomposition voltage (1.23 V) of water. Therefore, when a capacitor cell is assembled and then a voltage is applied for operation, moisture remaining in the pores will be electrolyzed to generate a gas. It has been found that the generated gas will gradually accumulate in pores of activated carbon and will remain in the interior of the element comprising electrodes facing each other with a separator interposed between them and impregnated with the electrolyte, without being discharged from the element.
If the capacitor is used for a long period of time under such a condition, the electrolyte in pores will be expelled by the generated gas, whereby the expected capacitance will not be obtained. Further, electrically conductive paths by ion transfer within pores will be cut off, thus leading to deterioration of the performance such as a decrease in the capacitance or an increase of the internal resistance.
Under these circumstances, it is an object of the present invention to solve the above-mentioned problems of the prior art and to provide a method for producing an electric double layer capacitor which is excellent in reliability of the performance for a long period of time and which has a high operation voltage and a high energy density.
The present invention provides a method for producing an electric double layer capacitor, which comprises impregnating an element comprising positive and negative electrodes facing each other with a separator interposed between them, with an organic electrolyte capable of forming an electric double layer on the surface of the electrodes to store electric charge, and then applying a voltage to the element, wherein said positive and negative electrodes are made of electrodes containing a carbonaceous material having a specific surface area of from 100 to 3,000 m2/g, and said organic electrolyte contains benzene or its chlorine derivative having at least one hydrogen atom of benzene substituted by a chlorine atom.
Now, the present invention will be described in detail with reference to the preferred embodiments.
In the present invention, benzene or its chlorine derivative having at least one hydrogen atom of benzene substituted by a chlorine atom (hereinafter referred to as xe2x80x9ca chlorine derivative of benzenexe2x80x9d) is represented by a compound of the formula 1. In the formula 1, n is an integer of from 1 to 6. 
In the present invention, the benzene or its chlorine derivative is preferably at least one member selected from the group consisting of benzene, monochlorobenzene, dichlorobenzene and trichlorobenzene. The structure of dichlorobenzene is not particularly limited, and any one of o-dichlorobenzene, m-dichlorobenzene p-dichlorobenzene may be preferably employed. Likewise, the structure of trichlorobenzene is not particularly limited.
In the present invention, the amount of the benzene or its chlorine derivative in the electrolyte may suitably be adjusted depending upon the amount of the remaining moisture or the pore characteristics of the carbonaceous material contained in the electrodes. It is preferred, however, that the benzene or its chlorine derivative is maintained to be completely dissolved in the organic electrolyte. Further, as the benzene or its chlorine derivative is added, the dielectric constant of the organic electrolyte decreases. Accordingly, the amount of the benzene or its chlorine derivative is preferably controlled within a range where a decrease in the ion conductivity due to a decrease in the dielectric constant of the organic electrolyte, is little. Thus, the amount is preferably from 0.1 to 30 wt %, particularly preferably from 1 to 10 wt %, in the electrolyte.
The operation mechanism in the present invention is not clearly understood, but it is considered that the benzene or its chlorine derivative contained in the electrolyte has high affinity to a pseudo-graphite surface present on the inner walls of pores of the carbonaceous material and is likely to be adsorbed in substitution with the water remaining in fine pores without being removed by the heat treatment. Thus, the remaining water can readily be expelled from the fine pores and discharged in the electrolyte outside the fine pores. When a voltage is applied to the element impregnated with the electrolyte, such water will readily be electrolyzed and gasified.
The storage of electric charge by formation of an electric double layer takes place mainly in the interior of pores. With a conventional electric double layer capacitor containing no benzene or chlorine derivative in the electrolyte, if a voltage is applied to the element impregnated with the electrolyte, a gas generated by the electrolysis of water remaining in the fine pores will remain in the fine pores, whereby the performance of the electric double layer capacitor tends to deteriorate. Whereas, in the present invention, as mentioned above, it is considered that the gas generated by the electrolysis by the remaining water by the application of a voltage, will be present outside the fine pores i.e. not in the fine pores. Namely, it is present in macro pores within the activated carbon particles or in spaces among the particles, or it will be discharged as bobbles out of the element. Thus, it is considered that deterioration of the performance of the electric double layer capacitor can be controlled to be little.
However, the decomposition gas generated by the application of a voltage to the element increases the inner pressure of the electric double layer capacitor cell. Accordingly, it is preferred that the application of the voltage during the production process is carried out in a dry atmosphere in an open condition to discharge the generated gas out of the capacitor cell. Here, the open condition is a condition where the element is not accommodated in a cell or even if it is accommodated in a cell, the cell is not closed.
Here, the dry atmosphere preferably has a dew point of not higher than xe2x88x9220xc2x0 C., more preferably not higher than xe2x88x9230xc2x0 C., most preferably not higher than xe2x88x9240xc2x0 C.
The voltage to be applied to the element is preferably at least 2 V which is higher than the decomposition voltage of water, more preferably at least 2.5 V. The temperature for the application of the voltage to the element is preferably from 15 to 85xc2x0 C., more preferably from 20 to 70xc2x0 C. If the voltage is applied under heating, the effect for increasing the durability of the electric double layer capacitor will be large, and the time for application of the voltage can be shortened. However, if the temperature is too high, the initial capacitance tends to decrease, and the internal resistance tends to increase.
Further, in the present invention, it is preferred to maintain the element under reduced pressure after the application of the voltage to the element, whereby the generated gas can more effectively be removed from the element. Here, the xe2x80x9creduced pressurexe2x80x9d is preferably a reduced pressure of at most 160 Torr, more preferably at most 10 Torr.
The application of a voltage to the element impregnated with the electrolyte is carried out usually by connecting the electrode constituting a positive electrode for the electric double layer capacitor (hereinafter referred to as the positive electrode of the element) to a plus terminal of a DC power source and connecting the electrode constituting a negative electrode of the electric double layer capacitor (hereinafter referred to as the negative electrode of the element) to a minus terminal of a DC power source. However, in order to further increase the durability of the electric double layer capacitor, it is preferred that this application of a voltage is repeated at least once reversing the connection of the positive electrode and the negative electrode. Namely, the application of a voltage to the element preferably comprises the following steps A and B:
Step A: a step of applying a voltage across the positive and negative electrodes by a DC power source, and
Step B: a step of applying a voltage by inversely connecting the positive and negative electrodes to the DC power source as compared with step A.
Here, either step A or step B may be carried out first, or step A and step B may be repeated alternately.
With respect to the electrodes in the present invention, both the positive electrode and the negative electrode contain a carbonaceous material having a specific surface area of from 100 to 3000 m2/g for forming an electric double layer at the interface with the electrolyte. As such a carbonaceous material, resin type activated carbon such as phenol, coconut shell type activated carbon, coke type activated carbon, pitch type activated carbon, carbon nanotube, carbon aerogel, carbon black or polyacene may, for example, be preferably used. The electrodes are preferably formed by adding an organic binder to at least one type of these carbonaceous materials. Particularly, they are preferably formed by adding an organic binder such as polytetrafluoroethylene or polyvinylidene fluoride to a carbonaceous material, followed by molding into a sheet, which is then bonded to a current collector, or by coating a liquid containing a carbonaceous material and the organic binder on a current collector.
In the present invention, the elute contained in the organic electrolyte to store electric charge by forming an electric double layer, is preferably a quaternary onium salt from the viewpoint of the electrical conductivity, the solubility and the electrochemical stability. Particularly, it preferably has a cation represented on R1R2R3R4N+ or R1R2R3R4P+, wherein each of R1, R2, R3 and R4 which are independent of one another, is a C1-6 alkyl group or a C6-10 aryl group. Particularly preferred is a salt comprising such a cation and an anion of e.g. BF4xe2x88x92, PF6xe2x88x92, CF3SO3xe2x88x92, ASF6xe2x88x92, N(SO2CF3)2xe2x88x92 or ClO4xe2x88x92.
The concentration of the above onium salt in the electrolyte is preferably at least 0.5 mol/l, more preferably at least 1.0 mol/l, for the purpose of securing the ion amount required for forming an electric double layer and for obtaining sufficient electrical conductivity.
The organic solvent to be used for the organic electrolyte is preferably at least one solvent selected from the group consisting of a cyclic carbonate such as ethylene carbonate, propylene carbonate or butylene carbonate, a linear carbonate such as dimethyl carbonate, ethyl methyl carbonate or diethyl carbonate, sulfolane, a sulfolane derivative and a nitrile such as acetonitrile or glutaronitrile.
The separator in the present invention is not particularly limited, but is preferably made of a porous material which is excellent in the electrical insulating property and in the chemical stability against the electrolyte and which is capable of absorbing a large amount of the electrolyte and excellent in the liquid-holding property. Specifically, it is preferably made of inorganic fibers such as glass fibers, silica fibers, alumina fibers or whiskers thereof, natural fibers such as manila hemp, or organic fibers such as synthetic fibers of e.g. polyolefin or polyester. It is preferably a sheet prepared from such fibers. Also preferred is a micro porous film having fine perforations formed by stretching a film made of e.g. polyolefin or polyester.
The structure of electric double layer capacitor of the present invention is not particularly limited and may preferably be any one of a coin structure wherein disk-shaped positive and negative electrodes face each other with a separator interposed therebetween and are impregnated with an electrolyte, a laminated structure wherein rectangular positive and negative electrodes are alternately laminated with a separator interposed therebetween, impregnated with an electrolyte and accommodated in an angular casing, and a winding structure wherein a pair of strip-shaped positive and negative electrodes are wound to face each other with a separator interposed therebetween, impregnated with an electrolyte and accommodated in a cylindrical casing.